A known obstacle detecting system includes plural ultrasonic sensors horizontally mounted on a bumper of a vehicle. According to this obstacle detecting system, a central axis in a transmitting direction of the ultrasonic sensors is set in a horizontal direction relative to the vehicle, and ultrasonic wave is transmitted normal to a surface (i.e., vertical to a surface) from the ultrasonic sensor mounted on a surface of the vehicle. By measuring time before receiving ultrasonic wave reflected back to the ultrasonic sensor by an obstacle, distance between the ultrasonic sensor and the obstacle is detected, and a driver is being alerted of the existence of that obstacle when the distance is within a predetermined range.
Ultrasonic sensors are classified into an ultrasonic sensor, which transmits ultrasonic wave, and this ultrasonic wave is being reflected back as an echo by a single ultrasonic sensor to detect obstacles, and a pair of ultrasonic sensors which detects obstacles by transmitting the ultrasonic wave by means of one of the sensors and by receiving echo by means of the other of the sensors. A two-dimensionally viewed detectable range of the single ultrasonic sensor, including the ultrasonic sensor, is sectorial. The two-dimensionally viewed detectable range of the pair of ultrasonic sensors including the ultrasonic sensor is elliptical.
With the construction of the known obstacle detecting system for the vehicle, for example disclosed in JP2001-208843A, various detectable ranges of obstacles can be set by combining the single ultrasonic sensor and the pair of ultrasonic sensors.
Notwithstanding, according to the known obstacle detecting system for the vehicle explained above, a central axis of ultrasonic waves in a transmitting direction is determined in a horizontal direction of the vehicle, and ultrasonic waves are transmitted normal to a surface of the vehicle on which the ultrasonic sensor is mounted. The ultrasonic sensor has undetectable ranges in the vicinity of a sensor head, and obstacles are undetectable in those ranges. Thus, there is a drawback that, in case an ultrasonic sensor is mounted on a bumper of a vehicle, when the vehicle nears an obstacle too close to the bumper, the obstacle becomes undetectable because of being in the undetectable range. There is another drawback that an obstacle located in an opening direction of a rear door of a vehicle becomes undetectable when a part of the rear door of the vehicle is off the detectable range of the obstacle by the ultrasonic sensor by the opening of the rear door because the ultrasonic sensor is mounted on the bumper.
A need thus exists for an obstacle detecting system for a vehicle, which improves an obstacle detecting performance in the vicinity of the vehicle.